Non-chromium coatings for aluminum

ABSTRACT

Composition and process for treating aluminum, aluminum alloys and anodized aluminum substrates to improve the corrosion resistance and adhesive bonding strength of the aluminum substrates. The process comprises pre- or post-treating said aluminum with an acidic aqueous solution comprising, per liter of solution, from about 0.1 to 22 grams of at least one fluorometallate, about 0.1 gram up to the solubility limit of a water soluble cationic or divalent zinc compound and, optionally, effective amounts of water soluble thickeners and/or surfactants.

ORIGIN OF INVENTION

The invention described herein was made by employee(s) of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to aqueous compositions and to the process ofusing said compositions for treating aluminum substrates with effectiveamounts of an acidic aqueous solution comprising at least onefluorometallate compound, cationic or divalent zinc compounds, watersoluble surfactants or wetting agents and water soluble thickeners. Thisinvention relates to compositions and to the process for treatingaluminum substrates to improve the adhesion bonding and corrosionresistant properties of the substrates. The process comprises treatingaluminum and its alloys, including anodized aluminum and its alloys witha composition comprising an acidic aqueous solution having a pH rangingfrom 2.5 to 5.5 which contains effective amounts of at least one watersoluble fluorometallate, at least one water soluble cationic or divalentzinc compound, and effective amounts of water soluble thickeners and/orsurfactants.

More specifically, this invention relates to non-chromium aqueouscompositions and the processes used to deposit coatings onto aluminum,aluminum alloys and anodized aluminum. These coatings are known asconversion coatings e.g. pretreatments or post-treatments designed toimprove paint adhesion, corrosion resistance, and electrical resistanceof the substrates. This invention also relates to the compositions andprocess used to deposit a film or coating onto aluminum and its alloysthat have been anodized and therefore have a porous surface thatrequires a post-treatment to enhance corrosion protection and avoiddegrading the adhesion-bonding characteristics of the anodized surface.Current conversion coatings or pretreatments for aluminum and aluminumalloys comprise hexavalent chromium or chromate chemistry. Hexavalentchromium is highly toxic and a known carcinogen. As a result, thecompositions used to deposit these coatings and the coatings per se aretoxic. However, these coatings provide outstanding paint adhesion, andcorrosion resistance, and can be applied by various methods includingimmersion, spray or wipe-on-techniques.

High performance post-treatments or sealers for anodized aluminum arebased also on hexavalent chromium chemistry. These coatings provideoutstanding paint adhesion and corrosion resistance to the anodizedaluminum. Typically, these coatings or films are deposited onto theanodized aluminum at elevated temperatures (over 50° Celsius) and areusually applied by immersion processes. These post-treatments generallyare required by the military and commercial users to followspecifications that govern each substrate being treated. As such thereis not a unique “post-treatment” specification for anodized aluminum asthere is for “conversion coating” aluminum.

Moreover, environmental laws, executive orders, and local occupational,safety, and health (OSHA) regulations are driving the military andcommercial users to search for chromate-free treatments. In the case ofanodized aluminum, the anodize film and base metal are relativelynon-toxic. With the addition of the chromate treatment, the coatingsbecome toxic. There are other coatings, however, such as nickel acetateand fluoride-containing compositions that do not contain chromium, buttheir technical performance is inferior to chromate-based coatings. Inaddition, the use of chromate treatments is becoming more expensive asregulations tighten. Costs can become prohibitive with future PEL(personal exposure limits) restrictions imposed by OSHA. Certainprocesses like spraying chromate-based solutions are forbidden at mostfacilities thereby forcing the use of less-than-optimum alternativeprocesses. Although, chromium compositions yield good corrosionprotection, certain DOD and commercial entities are seeking alternativecoatings that do not contain chromium. Even though chromate conversioncoatings and the chromate post-treatment of anodized aluminum provideoutstanding technical performance, from a life-cycle cost,environmental, and OSHA perspective, non-chromium alternatives arehighly desirable. Research is being directed to develop alternativecoatings for aluminum and its alloys that are technically equivalent oreven superior to chromium coatings. One important shortcoming of thepresent commercially available non-chromium coatings is the poorcorrosion-resistance properties. These non-chromium coatings generallyare inferior to treatments using hexavalent and trivalent chromium,showing little to no improvement in corrosion resistance compared to anuntreated aluminum substrate. There are no adequate non-chromiumcoatings available that offer the advantages of improved corrosionresistance, an adhesion base for subsequent coatings, non-chromiumformulations, practical color change, and robust processing capability.

SUMMARY OF THE INVENTION

This invention relates to chromium-free compositions and the process forpreparing corrosion-resistant coatings for aluminum, aluminum alloys,anodized aluminum and its alloys which comprises treating these aluminumsubstrates with an acidic aqueous solution comprising at least onefluorometallate compound, cationic or divalent zinc compounds, andoptionally thickeners such as cellulose-based thickeners andsurfactants. The compositions of this invention are referred to hereinas “NCP” for “Non-Chromium Post-Treatment” or “Non-Chromium Process”.

Therefore, it is an object of this invention to provide an acidicaqueous solution comprising fluorometallates such as thehexafluorozirconates, cationic or divalent zinc compounds for coatingsubstrates of aluminum, aluminum alloys, and anodized aluminum toimprove the adhesion and corrosion-resistance properties of thesubstrates.

It is another object of this invention to provide a stable acidicaqueous solution having a pH ranging from about 2.5 to 5.5 comprising analkali metal fluorometallate and cationic or divalent zinc salts forpost-treating anodized aluminum and aluminum alloys.

It is still another object of this invention to provide a treatment foraluminum substrates to provide color change and improved corrosionresistance.

It is still a further object of this invention to provide an acidicaqueous solution comprising fluorometallates and cationic or divalentzinc salt for treating aluminum and anodized aluminum substrates attemperatures ranging up to 212° F. wherein said acidic solution issubstantially free of chromium.

These and other object of the invention will become apparent byreference to the detailed description when considered in conjunctionwith the accompanying FIGS. 1-10 (photos).

DESCRIPTION OF THE DRAWINGS

FIG. 1 (photo) shows Alodine™ 1200S (chromate composition as control) on2024-T3 aluminum.

FIG. 2 (photo) shows Alodine™ 5700 (non-chromium composition as control)on 2024-T3 aluminum

FIG. 3 (photo) shows TCP (trivalent chromium composition as control) on2024-T3 aluminum.

FIG. 4 (photo) shows TCP with color change on 2024-T3 aluminum.

FIG. 5 (photo) shows non-chromium composition (NCP) on 2024-T3 aluminum.

FIG. 6 (photo) shows aluminum alloy (AA2024-T3) treated with Alodine™5700 for 24 hours in neutral salt fog test.

FIG. 7 (photo) shows aluminum alloy (AA2024-T3) treated with NCP-6 for24 hours in neutral salt fog test.

FIG. 8 (photo) shows aluminum alloy (AA2024-T3) treated with NCP-6 for 4days in neutral salt fog test.

FIG. 9 (photo) shows the corrosion results after treating aluminum panel(A) with composition (NCP-6) of this invention in comparison to panelstreated with a chromate composition (panel B), DI water (panel C), andan unsealed panel (D) after exposure to 1000 hours of neutral salt sprayfor 25 minutes at about 190° F.

FIG. 10 shows corrosion performance (ASTM D1654) of sealed, thin-filmsulfuric acid anodized aluminum alloy 2024 after 1000 hours of neutralsalt spray exposure in comparison to the compositions of this inventionand the prior U.S. patents.

DETAILED DESCRIPTION OF THE INTENTION

This invention relates to aqueous compositions and to the process ofusing said compositions which comprises an acidic aqueous solutionhaving a pH ranging from about 2.5 to 5.5, and preferably from about 3.7to 4.0 for treating aluminum, aluminum alloys, anodized aluminum andanodized aluminum alloys to improve the adhesion bonding (e.g. paintadhesion) and corrosion-resistance properties of said aluminums. Theprocess comprises preparing the pre- or post-treatments by using anacidic aqueous solution at temperatures ranging from ambient up to 160°F. or higher e.g. up to 212° F. The composition comprises, per liter ofsolution, from about 0.1 to 22 grams and preferably about 1.0 to 12.0grams e.g. 6.0 to 8.0 grams of at least one or more fluorometallateselected from the group consisting of hexafluorozirconates,tetrafluoroborates, hexafluorosilicates, hexafluorotitanates, andheptafluorotantalates, from about 0.1 up to the solubility limit andpreferably 0.1 to 12 grams of a cationic or divalent zinc compound, from0.0 to 10 grams and preferably from 0.5 to 10 grams of a water solublethickener, and from 0.0 to 10 grams and preferably 0.5 to 10 grams of asurfactant. The composition forms a coating on the aluminum that has apractical color change, imparts good paint adhesion and improves thecorrosion resistance of the aluminum.

More specifically, the post-treatments were applied to anodized aluminumsubstrates, for example, by the following method: Coupons were anodizedvia conventional anodizing techniques, immediately rinsed and thenimmersed directly into an aqueous composition such as Example 4. Couponswere allowed to dwell in NCP-6 (Example 4) for approximately 10 minutes,removed, and thoroughly rinsed in deionized water. Coupons were allowedto dry in a rack at ambient conditions. The aqueous compositions of thisinvention can be applied at about room temperature and applied to thealuminum substrate via immersion, spray or wipe on techniques. Solutiondwell time ranges from about 1 to 60 minutes depending on the anodizeprocess.

FIGS. 1-5 (photos) are examples of aluminum alloys coated with theacidic solutions of this invention compared to commercially availablechromium pretreatments and commercially available chromium-freepretreatments. Table I shows absolute color measurements of thepretreatment coatings compared to the trivalent chromium process (TCP).TABLE I Colorimetry comparing TCP, TCP with color, and NCP Matrix 02_25Colorimetry Results (all readings taken on Hunterlabs D25PC2Colorimeter) Alloy Pretreatment Panel L a b Alloy Pretreatment Panel L ab AA2519T87 TCP  9-1 52.1 −2.2 −4.9 AA7075T6 TCP 7-1 38.4 0.1 −3.7AA2519T87 TCP 19-29 51.2 −1.0 −4.1 AA7075T6 TCP 7-29 37.0 −0.2 −3.6AVERAGE 51.7 −1.6 −4.5 AVERAGE 37.7 −0.1 −3.7 AA2519T87 TCPcolor  9-841.4 −0.8 0.6 AA7075T6 TCPcolor 7-8 34.3 −0.4 −0.1 AA2519T87 TCPcolor 9-36 43.6 −0.7 4.2 AA7075T6 TCPcolor 7-36 33.8 −0.9 0.2 AVERAGE 42.5−0.8 2.4 AVERAGE 34.1 −0.7 0.1 AA2519T87 NCP6  9-15 38.3 −1.1 −1.4AA7075T6 NCP6 7-15 24.4 −0.8 −1.8 AA2519T87 NCP6  9-43 40.6 −1.6 −1.9AA7075T6 NCP6 7-43 25.0 −0.9 −2.1 AVERAGE 39.5 −1.4 −1.7 AVERAGE 24.7−0.9 −2.0 AA5083H131 TCP  3-1 62.5 −2.5 −0.1 AA2024T3 TCP 2-1 42.7 −0.3−7.1 AA5083H131 TCP  3-29 60.5 −2.5 0.6 AA2024T3 TCP 2-29 42.6 −0.4 −6.7AVERAGE 61.5 −2.5 0.3 AVERAGE 42.7 −0.4 −6.9 AA5083H131 TCPcolor  3-854.3 −2.2 0.1 AA2024T3 TCPcolor 2-8 34.9 −0.6 0.2 AA5083H131 TCPcolor 3-36 57.2 −3.4 0.9 AA2024T3 TCPcolor 2-36 36.0 −0.4 0.5 AVERAGE 55.8−2.8 0.5 AVERAGE 35.5 −0.5 0.4 AA5083H131 NCP6  3-15 40.8 −2.0 −1.2AA2024T3 NCP6 2-15 37.4 −0.4 −0.2 AA5083H131 NCP6  3-43 40.6 −1.6 −1.1AA2024T3 NCP6 2-43 37.4 −0.4 1.6 AVERAGE 40.7 −1.8 −1.2 AVERAGE 37.4−0.4 0.7

Table 2 provides examples of the performance of the compositions of thisinvention (NCP) in comparison to untreated aluminum alloys, alloystreated with commercially available hexavalent chromium pretreatments,trivalent chromium pretreatments (TCP), and commercially availablechromium-free pretreatments. In addition, Table 2 provides dry andwet-tape adhesion data for non-chromium post-treatment (NCP) compared totrivalent chromium control (TCP). The dry tape adhesion test (DTA) wasconducted in accordance with ASTM D3359, procedure A. The wet-tapeadhesion (WTA) test was conducted in accordance with ASTM D3359,procedure B. TABLE 2 Matrix Matrix 2_25 Paint All panels 3″ × 5″ ×0.032″ 2_25 Paint All panels 3″ × 5″ × 0.032″ Adhesion 24 hr dry beforeprimer, 14 day primer cure Adhesion 24 hr dry before primer, 14 dayprimer cure Alloy MIL- Alloy MIL- AA2024T3 PRF/C AA2219T87 PRF/C PanelCoating primer test results Panel Coating primer test results 2-2 TCP 5P85582nc DTA/WTA 1-day 5/4 19-2 TCP 5P 85582nc DTA/WTA 1-day 5/5 2-3 TCP5P 85582nc 4-day WTA 4 19-3 TCP 5P 85582nc 4-day WTA 5 2-4 TCP 5P85582nc 7-day WTA 5 19-4 TCP 5P 85582nc 7-day WTA 5 2-5 TCP 5P 53022DTA/WTA 1-day 5/5 19-5 TCP 5P 53022 DTA/WTA 1-day 5/5 2-6 TCP 5P 530224-day WTA 5 19-6 TCP 5P 53022 4-day WTA 5 2-7 TCP 5P 53022 7-day WTA 519-7 TCP 5P 53022 7-day WTA 5 2-16 NCP 6 85582nc DTA/WTA 1-day 5/5 19-16NCP 6 85582nc DTA/WTA 1-day 5/5 2-17 NCP 6 85582nc 4-day WTA 5 19-17 NCP6 85582nc 4-day WTA 5 2-18 NCP 6 85582nc 7-day WTA 4 19-18 NCP 6 85582nc7-day WTA 5 2-19 NCP 6 53022 DTA/WTA 1-day 5/5 19-19 NCP 6 53022 DTA/WTA1-day 5/5 2-20 NCP 6 53022 4-day WTA 5 19-20 NCP 6 53022 4-day WTA 52-21 NCP 6 53022 7-day WTA 5 19-21 NCP 6 53022 7-day WTA 5 2-29 TCP 5Pnone color 19-29 TCP 5P none color 2-30 TCP 5P 85582c DTA/WTA 1-day 5/419-30 TCP 5P 85582c DTA/WTA 1-day 5/5 2-31 TCP 5P 85582c 4-day WTA 419-31 TCP 5P 85582c 4-day WTA 5 2-32 TCP 5P 85582c 7-day WTA 5 19-32 TCP5P 85582c 7-day WTA 5 2-33 TCP 5P 23377 DTA/WTA 1-day 5/5 19-33 TCP 5P23377 DTA/WTA 1-day 5/5 2-34 TCP 5P 23377 4-day WTA 5 19-34 TCP 5P 233774-day WTA 5 2-35 TCP 5P 23377 7-day WTA 5 19-35 TCP 5P 23377 7-day WTA 52-44 NCP 6 85582c DTA/WTA 1-day 5/5 19-44 NCP 6 85582c DTA/WTA 1-day 5/52-45 NCP 6 85582c 4-day WTA 5 19-45 NCP 6 85582c 4-day WTA 5 2-46 NCP 685582c 7-day WTA 4 19-46 NCP 6 85582c 7-day WTA 5 2-47 NCP 6 23377DTA/WTA 1-day 5/5 19-47 NCP 6 23377 DTA/WTA 1-day 5/5 2-48 NCP 6 233774-day WTA 5 19-48 NCP 6 23377 4-day WTA 5 2-49 NCP 6 23377 7-day WTA 519-49 NCP 6 23377 7-day WTA 5 Matrix Matrix 2_25 Paint All panels 3″ ×5″ × 0.032″ 2_25 Paint All panels 3″ × 5″ × 0.032″ Adhesion 24 hr drybefore primer, 14 day primer cure Adhesion 24 hr dry before primer, 14day primer cure Alloy MIL- Alloy MIL- AA7075T6 PRF/C AA5083H131 PRF/CPanel Coating primer test results Panel Coating primer test results 7-2TCP 5P 85582nc DTA/WTA 1-day 5/5 3-2 TCP 5P 85582nc DTA/WTA 1-day 5/57-3 TCP 5P 85582nc 4-day WTA 5 3-3 TCP 5P 85582nc 4-day WTA 5 7-4 TCP 5P85582nc 7-day WTA 5 3-4 TCP 5P 85582nc 7-day WTA 5 7-5 TCP 5P 53022DTA/WTA 1-day 5/5 3-5 TCP 5P 53022 DTA/WTA 1-day 5/4 7-6 TCP 5P 530224-day WTA 5 3-5 TCP 5P 53022 4-day WTA 5 7-7 TCP 5P 85582nc 7-day WTA 53-7 TCP 5P 53022 7-day WTA 5 7-16 NCP 6 85582nc DTA/WTA 1-day 5/5 3-16NCP 6 85582nc DTA/WTA 1-day 5/5 7-17 NCP 6 85582nc 4-day WTA 5 3-17 NCP6 85582nc 4-day WTA 5 7-18 NCP 6 85582nc 7-day WTA 5 3-18 NCP 6 85582nc7-day WTA 5 7-19 NCP 6 53022 DTA/WTA 1-day 5/4 3-19 NCP 6 53022 DTA/WTA1-day 5/4 7-20 NCP 6 53022 4-day WTA 5 3-20 NCP 6 53022 4-day WTA 4 7-21NCP 6 53022 7-day WTA 5 3-21 NCP 6 53022 7-day WTA 5 7-30 TCP 5P 85582ncDTA/WTA 1-day 5/5 3-30 TCP 5P 85582nc DTA/WTA 1-day 5/5 7-31 TCP 5P85582nc 4-day WTA 5 3-31 TCP 5P 85582nc 4-day WTA 5 7-32 TCP 5P 85582nc7-day WTA 5 3-32 TCP 5P 85582nc 7-day WTA 5 7-33 TCP 5P 23377 DTA/WTA1-day 5/5 3-33 TCP 5P 23377 DTA/WTA 1-day 5/5 7-34 TCP 5P 23377 4-dayWTA 5 3-34 TCP 5P 23377 4-day WTA 5 7-35 TCP 5P 23377 7-day WTA 5 3-35TCP 5P 23377 7-day WTA 5 7-44 NCP 6 85582c DTA/WTA 1-day 5/5 3-44 NCP 685582c DTA/WTA 1-day 5/5 7-45 NCP 6 85582c 4-day WTA 5 3-45 NCP 6 85582c4-day WTA 5 7-46 NCP 6 85582c 7-day WTA 5 3-46 NCP 6 85582c 7-day WTA 57-47 NCP 6 23377 DTA/WTA 1-day 5/5 3-47 NCP 6 23377 DTA/WTA 1-day 5/57-48 NCP 6 23377 4-day WTA 5 3-48 NCP 6 23377 4-day WTA 5 7-49 NCP 623377 7-day WTA 5 3-49 NCP 6 23377 7-day WTA 5

FIGS. 6, 7 and 8 show the Corrosion Resistance of NCP for conversioncoated aluminum alloys compared to Alodine™ 5700. All panels wereprepared by the cleaning and deoxidizing process described herein, thenimmersed in either NCP or Alodine™ 5700 for 5 minutes. The panels wereracked and exposed in accordance with ASTM B117 for 1 to 7 days. Theserepresentative scans showed NCP-6 to have an equivalent or better barecorrosion performance after 4 days when compared to Alodine™ 5700 after24 hours. The Alodine™ 5700 panel was fully corroded after 2 daysexposure, and the NCP-6 panel was fully corroded after 7 days exposureto the salt fog test.

FIG. 9 shows the corrosion resistance results, after salt sprayexposure, of panel (A) treated with NCP-6 at ambient temperatures for 10minutes, panel (B) chromate treatment for 25 minutes at about 190° F.,panel (C) DI water treatment for 25 minutes at about 190° F., and panel(D) an unsealed panel.

FIG. 10 shows the corrosion performance of sealed, thin-film, sulfuricacid anodized aluminum alloy 2024 after 1000 hours of neutral salt sprayexposure. The panels compare the NCP-6 treatment of this invention withthe teachings of U.S. patents, with a chromate treatment, and with DIwater after 25 minutes at about 190° F.

In preparing the post-treatment for anodized aluminum, a unique featureis the ability to treat the anodized films without blocking the poresand still maintaining the excellent corrosion resistance. Generally,anodize aluminum coatings that are to be painted or adhesively bondedare not sealed by conventional methods, since the sealing process plugsthe pores of the coating and degrades paint adhesion andadhesive-bonding strengths. As a result, designers trade-off corrosionperformance for increased bonding performance. The treatments orcoatings of this invention allows the user to post-treat the anodizefilm to increase corrosion resistance while maintaining excellent paintadhesion. This is especially attractive, since there is an overallobjective to replace high-VOC, solvent borne chromated primers withlow-VOC, water borne non-chromated compositions. The post-treatment setforth in this invention provides added corrosion resistance to thecoating, potentially making the implementation of a more environmentallypreferred primer easier to accomplish while maintaining technicalperformance compared to the current systems. Another benefit is theimproved wear characteristics imparted to the anodize coating.Generally, all post-treatments or sealers soften or reduce the hardnessof anodize coatings. Chromate sealers are the least detrimental, whilewater sealing causes up to a 50% reduction in coating wear resistance.This softening effect is due to the hydration of the anodize coatingcaused by the sealers. Since the solutions of this invention can becarried out at about room temperature, little or no reduction in wearresistance occurs. This is especially important in anodizing where wearand corrosion resistance are critical.

The acidic solutions or compositions of this invention contain at leastone cationic or divalent zinc compound to provide color and to improvethe corrosion protection of the aluminum when compared to compositionsthat do not contain cationic zinc. The amount of the zinc compound canbe varied to adjust the color imparted to the coating, from as little asabout 0.1 gram per liter up to the solubility limit of the zinc compounddepending on the temperature of the solution e.g. 0.1 to 12 grams ofZinc²+cation. The cationic or divalent zinc can be supplied by variouschemical compounds e.g. salts that dissolves in water and are compatiblewith the other components in the acidic solution. Cationic or divalentzinc compounds that are water soluble at the required concentrationsi.e. up to the solubility limits of the compounds, preferably include,zinc acetate, zinc telluride, zinc tetrafluoroborate, zinc molybdate,zinc hexafluorosilicate, zinc sulfate and various other zinc salts andany combination thereof in any ratio.

The pretreatment or post-treatment of the aluminum can be carried out atvarious temperatures including the temperature of the solution whichranges from ambient e.g. from about room temperature up to about 160° F.or higher e.g. up to about 212° F. Room temperature up to 100° F. ispreferred, however, in that this eliminates the necessity for heatingequipment. The subsequent coating may be air dried by any of the methodsknown in the art including, for example, oven drying, forced-air drying,exposure to infra-red lamps, and the like. For purposes of thisinvention, the term aluminum includes alloys of aluminum containingsmall but effective amounts of various other metals.

The following Examples illustrate the stable solutions of thisinvention, and the method of using the acidic solutions in providingcolor recognition, improved adhesion bonding and corrosion-resistant foraluminum, aluminum alloy and anodized aluminum substrates.

EXAMPLE 1

A stable acidic aqueous solution having a pH ranging from about 3.7 to4.0 for post-treating an anodized aluminum to provide acorrosion-resistant coating thereon comprises, per liter of solution,from about 1.0 to 12 grams of potassium hexafluorozirconate, about 0.1to 12 grams of zinc sulfate and from about 0.01 to 10 grams of anorganic thickener.

EXAMPLE 2

A stable acidic aqueous solution of deionized water for treating analuminum alloy substrate to form a corrosion-resistant coating thereoncomprises, per liter of solution, about 4.0 grams of potassiumhexafluorozirconate, about 3.0 grams of zinc sulfate, and about 1.0 gramof methyl cellulose.

EXAMPLE 3

A NCP solution was prepared by mixing about 10 grams per liter ofsolution of potassium hexafluorozirconate, about 10 grams per liter of awater soluble divalent zinc salt and about 0.5 grams of cellulose.

EXAMPLE 4

An NCP-6 solution was prepared by mixing 4.0 grams per liter ofpotassium hexafluorozirconate and 3.0 grams per liter of zinc sulfate indeionized water. The NCP-6 solution can then be used as is or diluted asappropriate.

EXAMPLE 5

A sprayable concentrate of NCP was produced by mixing 8.0 grams perliter of potassium hexafluorozirconate and 6.0 grams per liter of zincsulfate in deionized water. About 0.1 gram per liter of Methocel F4M canbe added to enhance wetting and reduce evaporation of water duringspraying.

More specifically, the conversion coatings were applied onto 2024-T3aluminum as follows: Test coupons made from 2024-T3 aluminum werecleaned in a standard alkaline cleaner (Turco NC-LT) at 120° F. for 20minutes. Coupons were double rinsed in warm tap water and immersed inTuco Smut-Go NC for 5 minutes. Coupons were double rinsed in cold tapwater and then immediately (without drying) immersed into thecomposition set forth in Example 4. Coupons were allowed to dwell in theNCP-6 for approximately 10 minutes, removed, and thoroughly rinsed indeionized water. Coupons were then allowed to dry in a rack at ambientconditions. The resulting coating was a dark brown/tan in color, easilyvisible from across the laboratory. This is critical for quality controlduring processing so that processors have a way of knowing that acoating is present. This process can be used for any aluminum to producean acceptable conversion coating with variations on immersion timesdepending on the aluminum or alloy leading to various color intensities,shades and coating weights.

In preparing the acidic solutions of this invention, water solublesurfactants can be added to the acidic solutions in amounts ranging fromabout 0.0 to 10 or 0.5 to 10 grams per liter and preferably from about0.5 to 1.5 grams or 1.0 gram per liter. The surfactants are added to theaqueous solution to provide better wetting properties by lowering thesurface tension thereby insuring complete coverage, and a more uniformfilm on the aluminum substrates. The surfactants include at least onewater soluble compound selected from the group consisting of non-ionic,anionic, and cationic surfactants. These surfactants include themonocarboxyl imidoazolines, alkylsulfate sodium salts (DUPONOL®),tridecyloxypoly(alkyleneoxy ethanol), ethoxylated or propoxylatedalkylphenols (IGEPAL®), alkylsulfonamides, alkaryl sulfonates, thealkylaryl polyether alcohols such as octylphenoxypolyethoxy ethanol(TRITON®), sorbitan monopaimitate (SPAN®), polyoxyethylenealkylphenylethers, dodecylphenyl polyethyleneglycol ether (TERGITOL®), alkylpyrrolidones, polyalkoxylated fatty acid esters, alkylbenzene sulfonatesand mixtures thereof. Other water soluble surfactants include thealkylphenol alkoxylates, preferably the nonylphenol ethoxylates, andadducts of ethylene oxide with fatty amines; see the publication:“Surfactants and Detersive Systems”, published in Kirk-Othmer'sEncyclopedia of Chemical Technology, 3^(rd) Ed.

When large surfaces do not permit immersion or where vertical surfacesare to be sprayed, thickening agents can be added to retain the aqueoussolution on the surface for sufficient contact time. The thickenersemployed are preferably the organic water soluble thickeners added tothe coating solutions at sufficient concentrations ranging from about0.0 to 10 grams or 0.5 to 10 grams per liter and preferably 0.5 to 1.5grams or 1.0 gram per liter of the acidic solution. Specific examples ofsome preferred thickeners include the cellulose compounds, such ashydroxypropyl cellulose (Klucel), methyl or ethyl cellulose,hydroxyethyl cellulose, hydroxymethyl cellulose and mixtures thereof.Other water soluble inorganic thickeners include colloidal silica, clayssuch as bentonite, starches, gum arabic, tragacanth, agar and variouscombinations.

The aqueous solution can be applied to the aluminum substrate viaimmersion, spray or wipe-on techniques. The solutions can be used attemperatures ranging from about ambient up to about 160° F. or highere.g. from about room temperature up to about 212° F. and optimallyapplied via immersion to further improve the corrosion resistance of thecoatings. Solution dwell time ranges from about 1 to 10 minutes and ashigh as 60 minutes or more, and preferably, for example, from about 5 to15 minutes at about 80° F. After dwelling, the remaining solution isthen thoroughly rinsed from the alloy with tap or deionized water. Noadditional chemical manipulations of the deposited films are necessaryfor excellent performance. In addition, the aqueous solutions may besprayed from a spray tank apparatus designed to replace immersion tanks.This concept also reduces active chemical volume from about 1,000gallons to about 30 to 50 gallons.

While this invention has been described by a number of specificexamples, it is obvious that there are other variations andmodifications which can be made without departing from the spirit andscope of the invention as particularly set forth in the appended claims.

1. Process for preparing coatings on aluminum to improve the adhesivebonding and corrosion resistance which comprises treating the aluminumwith an acidic aqueous solution having a pH ranging from about 2.5 to5.5 at temperatures ranging from ambient up to about 212° F.; saidacidic aqueous solution comprising, per liter of solution, from about0.1 to 22 grams of at least one fluorometallate selected from the groupconsisting of hexafluorozirconates, tetrafluoroborates,hexafluorosilicates, hexafluorotitanates, and heptafluorotantalates,from about 0.1 up to the solubility limit of at least one cationic zinccompound, from about 0.0 to 10 grams of a water soluble thickener, and0.0 to 10 grams of a water soluble surfactant.
 2. The process of claim 1wherein the aluminum is an alloy of aluminum.
 3. The process of claim 1wherein the aluminum is anodized aluminum.
 4. The process of claim 1wherein the aluminum is an anodized aluminum alloy.
 5. The process ofclaim 1 wherein the pH of the acidic aqueous solution ranges from about3.7 to 4.0.
 6. The process of claim 5 wherein the fluorometallate is ahexafluorozirconate.
 7. The process of claim 6 wherein the zinc compoundis zinc sulfate and the hexafluorozirconate is an alkali metalzirconate.
 8. The process of claim 7 wherein the aqueous solutioncontains from about 0.5 to 10 grams of a cellulose thickener.
 9. Theprocess of claim 8 wherein the aqueous solution contains from about 0.5to 10 grams of a non-ionic surfactant.
 10. Process for preparingcoatings on aluminum to improve the adhesive bonding and corrosionresistance of the aluminum which comprises treating the aluminum with anacidic aqueous solution having a pH ranging from about 3.7 to 4.0 attemperatures ranging from ambient up to about 160° F.; said acidicaqueous solution comprising, per liter of solution, from about 1.0 to 12grams of an alkali metal fluorometallate, from about 0.1 up to thesolubility limits of at least one cationic zinc salt, from 0.5 to 1.5grams of a water soluble thickener and from 0.5 to 1.5 grams of a watersoluble surfactant.
 11. The process of claim 10 wherein the thickener isa cellulose compound and the surfactant is a non-ionic surfactant. 12.The process of claim 11 wherein the cellulose compound is methylcellulose.
 13. The process of claim 10 wherein the zinc salt is zincsulfate and the fluorometallate is a hexafluorozirconate.
 14. Theprocess of claim 13 wherein the zirconate is potassiumhexafluorozirconate.
 15. The process of claim 14 wherein the potassiumhexafluorozirconate ranges from about 1.0 to 12 grams and the zinc saltranges from about 0.1 to 12 grams.
 16. The process of claim 14 whereinthe zinc salt is zinc acetate.
 17. The coated aluminum obtained by theprocess of claim
 1. 18. Compositions for improving the adhesion bondingand corrosion resistance of aluminum, aluminum alloy, and anodizedaluminum which comprise an acidic aqueous solution having a pH rangingfrom about 2.5 to 5.5, and per liter of solution, from about 0.1 to 22grams of at least one fluorometallate selected from the group consistingof hexafluorozirconates, tetrafluoroborates, hexafluorosilicates,hexafluorotitanates, and heptafluorotantalates, from about 0.1 up to thesolubility limit of a cationic zinc compound, from about 0.0 to 10 gramsof at least one water soluble thickener and from 0.0 to 10 grams of atleast one water soluble surfactant.
 19. The composition of claim 18wherein the pH of the aqueous solution ranges from about 3.7 to 4.0. 20.The composition of claim 19 wherein the fluorometallate is ahexafluorozirconate ranging from about 1.0 to 12 grams and the zinccompound is a divalent zinc salt ranging from about 0.1 to 12 grams perliter of solution.
 21. The composition of claim 20 wherein the thickeneris a cellulose compound ranging from about 0.5 to 10 grams and thesurfactant is a non-ionic surfactant ranging from about 0.5 to 10 gramsper liter of solution.